Controlled solvent vapor annealing of a high χ block copolymer thin film

Disclosing Topographical and Chemical Patterns in Confined Films of High-Molecular-Weight Block Copolymers under Controlled Solvothermal Annealing

The microphase separation of high-molecular-weight block copolymers into nanostructured films is strongly dependent on the surface fields. Both, the chain mobility and the effective interaction parameters can lead to deviations from the bulk morphologies in the structures adjacent to the substrate. Resolving frustrated morphologies with domain period L0 above 100 nm is an experimental challenge. Here, solvothermal annealing was used to assess the contribution of elevated temperatures of the vapor Tv and of the substrate Ts on the evolution of the microphase-separated structures in thin films symmetric of polystyrene-b-poly(2vinylpyridine) block copolymer (PS-PVP) with L0 about 120 nm. Pronounced topographic mesh-like and stripe patterns develop on a time scale of min and are attributed to the perforated lamella (PL) and up-standing lamella phases. By setting Tv/Ts combinations it is possible to tune the sizes of the resulting PL patterns by almost 10%. Resolving chemical periodicity using selective metallization of the structures revealed multiplication of the topographic stripes, i.e., complex segregation of the component within the topographic pattern, presumably as a result of morphological phase transition from initial non-equilibrium spherical morphology. Reported results reveal approaches to tune the topographical and chemical periodicity of microphase separation of high-molecular-weight block copolymers under strong confinement, which is essential for exploiting these structures as functional templates.

Preparation, Properties, and Bioapplications of Block Copolymer Nanopatterns

Block copolymer (BCP) self-assembly has emerged as a feasible method for large-scale fabrication with remarkable precision - features that are not common for most of the nanofabrication techniques. In this review, recent advancements in the molecular design of BCP along with state-of-the-art processing methodologies based on microphase separation alone or its combination with different lithography methods are presented. Furthermore, the bioapplications of the generated nanopatterns in the development of protein arrays, cell-selective surfaces, and antibacterial coatings are explored. Finally, the current challenges in the field are outlined and the potential breakthroughs that can be achieved by adopting BCP approaches already applied in the fabrication of electronic devices are discussed.

Morphology Engineering of the Asymmetric PS-b-P4VP Block Copolymer: From Porous to Nanodot Oxide Structures

In the present work, we demonstrate the formation of oxide porous and nanodot structures from the same block copolymer (BCP) by the phase inversion of a BCP template. We investigated the effect of solvent annealing time on the ordering of asymmetric, cylinder forming, polystyrene-b-poly(4-vinylpyridine) (PS-b-P4VP) BCP. Phase separation of PS-b-P4VP was achieved by solvent vapor annealing (SVA) in a solvent atmosphere that is (partially) selective to P4VP to initially generate hexagonally arranged, cylindrical arrays of the expected structure. The morphology of the BCP changed from P4VP hexagonally packed cylinders to an 'inverse' structure with PS cylinders embedded in a P4VP matrix. This suggests that selective swelling occurs over time such that the swollen P4VP phase becomes the majority volume component. Metal ions (Ga3+, In3+) were infiltrated into the BCP templates by a solution-mediated infiltration approach, followed by an ultraviolet-ozone treatment to remove the polymer and oxidize the metallic ions to their oxides. The findings show that a single BCP can be used to create both metal oxide arrays and porous structures of metal oxides by simply varying the duration of the solvent annealing process. The resulting structures were analyzed through several methods including scanning electron microscopy, atomic force microscopy, X-ray photoelectron spectroscopy (XPS), transmission electron microscopy, and energy-dispersive X-ray spectroscopy. XPS analyses confirmed the complete elimination of the BCP template and the presence of metal oxides. This study provides important insights into the development of functional BCP materials with inverse structures.

Horizontal to perpendicular transition of lamellar and cylinder phases in block copolymer films induced by interface segregation of single-chain nanoparticles during solvent evaporation

How to fabricate perpendicularly oriented domains (PODs) of lamellar and cylinder phases in block copolymer thin films remains a major challenge. In this work, via a coarse-grained molecular dynamics simulation study, we report a solvent evaporation strategy starting from a mixed solution of A-b-B-type diblock copolymers (DBCs) and single-chain nanoparticles (SCNPs) with the same composition, which is capable of spontaneously generating PODs in drying DBC films induced by the interface segregation of SCNPs. The latter occurs at both the free surface and substrate and, consequently, neutralizes the interface selectivity of distinct blocks in DBCs, leading to spontaneous formation of PODs at both interfaces. The interface segregation of SCNPs is related to the weak solvophilicity of the internal cross-linker units. A mean-field theory calculation demonstrates that the increase in the chemical potential of SCNPs in the bulk region drives their interface segregation along with solvent evaporation. We believe that such a strategy can be useful in regulating the PODs of DBC films in practical applications.

Solvent-assisted self-assembly of block copolymer thin films

Solvent-assisted block copolymer self-assembly is a compelling method for processing and advancing practical applications of these materials due to the exceptional level of the control of BCP morphology and significant acceleration of ordering kinetics. Despite substantial experimental and theoretical efforts devoted to understanding of solvent-assisted BCP film ordering, the development of a universal BCP patterning protocol remains elusive; possibly due to a multitude of factors which dictate the self-assembly scenario. The aim of this review is to aggregate both seminal reports and the latest progress in solvent-assisted directed self-assembly and to provide the reader with theoretical background, including the outline of BCP ordering thermodynamics and kinetics phenomena. We also indicate significant BCP research areas and emerging high-tech applications where solvent-assisted processing might play a dominant role.

Poly(vinylpyridine)-Containing Block Copolymers for Smart, Multicompartment Particles

Multicompartment particles generated by the self-assembly of block copolymers (BCPs) have received considerable attention due to their unique morphologies and functionalities. A class of important building blocks for multicomponent particles...

Defining Swelling Kinetics in Block Copolymer Thin Films: The Critical Role of Temperature and Vapour Pressure Ramp

We studied the kinetics of swelling in high-χ lamellar-forming poly(styrene)-block- poly(lactic acid) (PS-b-PLA) block copolymer (BCP) by varying the heating rate and monitoring the solvent vapour pressure and the substrate temperature in situ during solvo-thermal vapour annealing (STVA) in an oven, and analysing the resulting morphology. Our results demonstrate that there is not only a solvent vapour pressure threshold (120 kPa), but also that the rate of reaching this pressure threshold has a significant effect on the microphase separation and the resulting morphologies. To study the heating rate effect, identical films were annealed in a tetrahydrofuran (THF) vapour environment under three different ramp regimes, low (rT<1 °C/min), medium (24 °C/min), for 60, 90 and 120 min, respectively, while the solvent vapour pressure and the substrate temperature were measured in real time. The translational order improved significantly with increasing the heating rate. The solvent mass uptake calculated for the different ramp regimes during annealing is linearly proportional to time, indicating that the swelling kinetics followed Case II diffusion. Two stages of the swelling behaviour were observed: (i) diffusion at the initial stages of swelling and (ii) stress relaxation, controlled at later stages. Films with a faster rate of increase in vapour pressure (rP>2 kPa/min) reached the pressure threshold value at an early stage of the swelling and attained a good phase separation. According to our results, highly ordered patterns are only obtained when the volume fraction of the solvent exceeds the polymer volume fraction, i.e., (φs≥φp), during the swelling process, and below this threshold value (φs=0.5), the films did not obtain a good structural order, even at longer annealing times.

Fabrication of Nanoscale Oxide Textured Surfaces on Polymers

Nanoscale textured surfaces play an important role in creating antibacterial surfaces, broadband anti-reflective properties, and super-hydrophobicity in many technological systems. Creating nanoscale oxide textures on polymer substrates for applications such as ophthalmic lenses and flexible electronics imposes additional challenges over conventional nanofabrication processes since polymer substrates are typically temperature-sensitive and chemically reactive. In this study, we investigated and developed nanofabrication methodologies to create highly ordered oxide nanostructures on top of polymer substrates without any lithography process. We developed suitable block copolymer self-assembly, sequential infiltration synthesis (SIS), and reactive ion etching (RIE) for processes on polymer substrates. Importantly, to prevent damage to the temperature-sensitive polymer and polymer/oxide interface, we developed the process to be entirely performed at low temperatures, that is, below 80 °C, using a combination of UV crosslinking, solvent annealing, and modified SIS and RIE processes. In addition, we developed a substrate passivation process to overcome reactivity between the polymer substrate and the SIS precursors as well as a high precision RIE process to enable deep etching into the thermally insulated substrate. These methodologies widen the possibilities of nanofabrication on polymers.

Green Nanofabrication Opportunities in the Semiconductor Industry: A Life Cycle Perspective

The turn of the 21st century heralded in the semiconductor age alongside the Anthropocene epoch, characterised by the ever-increasing human impact on the environment. The ecological consequences of semiconductor chip manufacturing are the most predominant within the electronics industry. This is due to current reliance upon large amounts of solvents, acids and gases that have numerous toxicological impacts. Management and assessment of hazardous chemicals is complicated by trade secrets and continual rapid change in the electronic manufacturing process. Of the many subprocesses involved in chip manufacturing, lithographic processes are of particular concern. Current developments in bottom-up lithography, such as directed self-assembly (DSA) of block copolymers (BCPs), are being considered as a next-generation technology for semiconductor chip production. These nanofabrication techniques present a novel opportunity for improving the sustainability of lithography by reducing the number of processing steps, energy and chemical waste products involved. At present, to the extent of our knowledge, there is no published life cycle assessment (LCA) evaluating the environmental impact of new bottom-up lithography versus conventional lithographic techniques. Quantification of this impact is central to verifying whether these new nanofabrication routes can replace conventional deposition techniques in industry as a more environmentally friendly option.

Optimization and Control of Large Block Copolymer Self-Assembly via Precision Solvent Vapor Annealing

The self-assembly of ultra-high molecular weight (UHMW) block copolymers (BCPs) remains a complex and time-consuming endeavor owing to the high kinetic penalties associated with long polymer chain entanglement. In this work, we report a unique strategy of overcoming these kinetic barriers through precision solvent annealing of an UHMW polystyrene-block-poly(2-vinylpyridine) BCP system (M w: ∼800 kg/mol) by fast swelling to very high levels of solvent concentration (ϕs). Phase separation on timescales of ∼10 min is demonstrated once a thickness-dependent threshold ϕs value of ∼0.80-0.86 is achieved, resulting in lamellar feature spacings of over 190 nm. The threshold ϕs value was found to be greater for films with higher dry thickness (D 0) values. Tunability of the domain morphology is achieved through controlled variation of both D 0 and ϕs, with the kinetically unstable hexagonal perforated lamellar (HPL) phase observed at ϕs values of ∼0.67 and D 0 values of 59-110 nm. This HPL phase can be controllably induced into an order-order transition to a lamellar morphology upon further increase of ϕs to 0.80 or above. As confirmed by grazing-incidence small-angle X-ray scattering, the lateral ordering of the lamellar domains is shown to improve with increasing ϕs up to a maximum value at which the films transition to a disordered state. Thicker films are shown to possess a higher maximum ϕs value before transitioning to a disordered state. The swelling rate is shown to moderately influence the lateral ordering of the phase-separated structures, while the amount of hold time at a particular value of ϕs does not notably enhance the phase separation process. These large period self-assembled lamellar domains are then employed to facilitate pattern transfer using a liquid-phase infiltration method, followed by plasma etching, generating ordered, high aspect ratio Si nanowall structures with spacings of ∼190 nm and heights of up to ∼500 nm. This work underpins the feasibility of a room-temperature, solvent-based annealing approach for the reliable and scalable fabrication of sub-wavelength nanostructures via BCP lithography.

Large-Area Fabrication of Vertical Silicon Nanotube Arrays via Toroidal Micelle Self-Assembly

We present a highly scalable, room-temperature strategy for fabricating vertical silicon nanotube arrays derived from a toroidal micelle pattern via a water vapor-induced block copolymer (BCP) self-assembly mechanism. A polystyrene-b-poly(ethylene oxide) (PS-b-PEO) BCP system can be self-assembled into toroidal micelle structures (diameter: 400-600 nm) on a PS-OH-modified substrate in a facile manner contrasting with other complex processes described in the literature. It was found that a minimum PS-b-PEO thickness of ∼86 nm is required for the toroidal self-assembly. Furthermore, a water vapor annealing treatment at room conditions (∼25 °C, 60 min) is shown to vastly enhance the ordering of micellar structures. A liquid-phase infiltration process was used to generate arrays of iron and nickel oxide nanorings. These oxide structures were used as templates for pattern transfer into the underlying silicon substrate via plasma etching, resulting in large-area 3D silicon nanotube arrays. The overall simplicity of this technique, as well as the wide potential versatility of the resulting metal structures, proves that such room-temperature synthesis routes are a viable pathway for complex nanostructure fabrication, with potential applicability in fields such as optics or catalysis.

Large-Grained Cylindrical Block Copolymer Morphologies by One-Step Room-Temperature Casting

We report a facile method of ordering block copolymer (BCP) morphologies in which the conventional two-step casting and annealing steps are replaced by a single-step process where microphase separation and grain coarsening are seamlessly integrated within the casting protocol. This is achieved by slowing down solvent evaporation during casting by introducing a nonvolatile solvent into the BCP casting solution that effectively prolongs the duration of the grain-growth phase. We demonstrate the utility of this solvent evaporation annealing (SEA) method by producing well-ordered large-molecular-weight BCP thin films in a total processing time shorter than 3 min without resorting to any extra laboratory equipment other than a basic casting device, i.e., spin- or blade-coater. By analyzing the morphologies of the quenched samples, we identify a relatively narrow range of polymer concentration in the wet film, just above the order-disorder concentration, to be critical for obtaining large-grained morphologies. This finding is corroborated by the analysis of the grain-growth kinetics of horizontally oriented cylindrical domains where relatively large growth exponents (1/2) are observed, indicative of a more rapid defect-annihilation mechanism in the concentrated BCP solution than in thermally annealed BCP melts. Furthermore, the analysis of temperature-resolved kinetics data allows us to calculate the Arrhenius activation energy of the grain coarsening in this one-step BCP ordering process.

Strategy for Enhancing Ultrahigh-Molecular-Weight Block Copolymer Chain Mobility to Access Large Period Sizes (>100 nm)

Assembling ultrahigh-molecular-weight (UHMW) block copolymers (BCPs) in rapid time scales is perceived as a grand challenge in polymer science due to slow kinetics. Through surface engineering and identifying a nonvolatile solvent (propylene glycol methyl ether acetate, PGMEA), we showcase the impressive ability of a series of lamellar poly(styrene)-block-poly(2-vinylpyridine) (PS-b-P2VP) BCPs to self-assemble directly after spin-coating. In particular, we show the formation of large-period (≈111 nm) lamellar structures from a neat UHMW PS-b-P2VP BCP. The significant influence of solvent-polymer solubility parameters are explored to enhance the polymer chain mobility. After optimization using solvent vapor annealing, increased feature order of ultralarge-period PS-b-P2VP BCP patterns in 1 h is achieved. Isolated metallic and dielectric features are also demonstrated to exemplify the promise that large BCP periods offer for functional applications. The methods described in this article center on industry-compatible patterning schemes, solvents, and deposition techniques. Thus, our straightforward UHMW BCP strategy potentially paves a viable and practical path forward for large-scale integration in various sectors, e.g., photonic band gaps, polarizers, and membranes that demand ultralarge period sizes.

Precise Definition of a "Monolayer Point" in Polymer Brush Films for Fabricating Highly Coherent TiO2 Thin Films by Vapor-Phase Infiltration

In this work, we show that in order to fabricate coherent titania (TiO₂) films with precise thickness control, it is critical to generate a complete polymer brush monolayer. To date, demonstrations of such dense polymer monolayer formation that can be utilized for inorganic infiltration have been elusive. We describe a versatile bottom-up approach to covalently and rapidly (60 s processing) graft hydroxyl-terminated poly(2-vinyl pyridine) (P2VP-OH) polymers on silicon substrates. P2VP-OH monolayer films of varying thicknesses can subsequently be used to fabricate high-quality TiO₂ films. Our innovative strategy is based upon room-temperature titanium vapor-phase infiltration of the grafted P2VP-OH polymer brushes that can produce TiO₂ nanofilms of 2-4 nm thicknesses. Crucial parameters are explored, including molecular weight and solution concentration for grafting dense P2VP-OH monolayers from the liquid phase with high coverage and uniformity across wafer-scale areas (>2 cm²). Additionally, we compare the P2VP-OH polymer systems with another reactive polymer, poly(methyl methacrylate)-OH, and a relatively nonreactive polymer, poly(styrene)-OH. Furthermore, we prove the latter to be effective for surface blocking and deactivation. We show a simple process to graft monolayers for polymers that are weakly interacting with one another but more challenging for reactive systems. Our methodology provides new insight into the rapid grafting of polymer brushes and their ability to form TiO₂ films. We believe that the results described herein are important for further expanding the use of reactive and unreactive polymers for fields including area-selective deposition, solar cell absorber layers, and antimicrobial surface coatings.

Polystyrene-block-poly(ethylene oxide) Thin Films Fabricated from a Solvent Mixture for the Co-Assembly of Polymers and Proteins

The co-assembly of peptides and proteins in poly(styrene-block-ethylene oxide) (PS-b-PEO) thin films has proven to be a promising method to fabricate polymer-biomolecule functional materials. Contrary to the covalent immobilization of biomolecules on surfaces, co-assembly presents the opportunity to arrange cargo within thin films, which can be released upon exposure to an aqueous environment. The use of a mixed solvent system ensures the solubilization of hydrophobic polymer as well as the solubilization and protection of the biomolecule cargo. However, to produce largely defect-free films of PS-b-PEO from a solvent mixture containing water is challenging due to the narrow range of solvent miscibility and polymer/protein solubility. This work explores the limits of using a benzene/methanol/water solvent mixture for the production of thin PS-b-PEO films and provides a template for the fabrication optimization of block copolymer thin films in different complex solvent systems. The film quality is analyzed using optical microscopy and atomic force microscopy and correlated to the solvent composition. By adjusting the solvent composition to 80/18.8/1.2 vol % benzene/methanol/water, it was possible to reliably fabricate thin films with less than 1% macroscopic defect surface coverage. Using the optimized solvent composition, we also demonstrate the fabrication of ordered PS-b-PEO films containing lysozyme. Furthermore, we show the release of lysozyme into aqueous media, which highlights the potential use of such films for drug delivery applications.

Perpendicularly aligned nanodomains on versatile substrates via rapid thermal annealing assisted by liquid crystalline ordering in block copolymer films

The highly ordered perpendicularly aligned cylindrical and lamellar microdomains within block copolymer (BCP) films have important applications in diverse fields. However, the fast normal orientation of self-assembled nanostructures on arbitrary substrates without tedious pre- and postprocessing has been a challenging issue in manufacturing miniaturized devices. Here, we outline the potential for extending the hierarchical self-assembly within azobenzene-containing PS-b-PMA(Az) films to inherently assist in the formation of normally aligned domains using a rapid thermal annealing process (140 °C for 5 min). Liquid crystalline (LC) mesogens in PS-b-PMA(Az) films self-assemble to form a parallelly aligned sematic phase after thermal annealing, as confirmed by grazing-incidence small-angle X-ray scattering (GISAXS), wide-angle X-ray diffraction (WAXD) and ultraviolet-visible (UV-vis) spectra. This sub-phase contributes to broadening of the PS-cylinder-phase window (0.083 ≤ f _PS < 0.49) and ∼12 nm PS cylinder structures. Perpendicular cylinders or lamellae are observed on various substrates, such as silicon wafers, flexible polyethylene terephthalate (PET) sheets and conductive aluminum foils. Additionally, the good reactive ion etching (RIE) rate difference between the two blocks makes these BCPs more attractive for advancing the field of BCP lithographic applications for fabricating flexible microelectronic devices.

Enhanced microphase separation of thin films of low molecular weight block copolymer by the addition of an ionic liquid

We report on the use of a selective, non-volatile ionic liquid (IL) to enhance the self-assembly via solvent annealing of a low molecular weight block copolymer (BCP) of styrene and 2-vinylpyridine (2VP) suitable for generating sub-10 nm features. Diblock and triblock copolymers of different molecular weights of styrene and 2VP are individually blended with the IL and then solvent annealed in acetone, a non-preferential solvent for the BCPs. Differential scanning calorimetry indicates that the IL selectively resides in the 2VP block of the BCP, resulting in a decrease of the block's Tg and an increase of the effective Flory-Huggins parameter (χeff) of the BCP. The influence of the IL on the non-preferential window of a random copolymer brush used to treat the substrate for self-assembly of the BCPs is also analyzed. Well-defined lamellar patterns form when the optimal weight ratio of IL (∼1%) is added to the BCPs. A detailed analysis of the orientational correlation length and pitch size of the BCPs quantitatively shows that the addition of the IL enhanced the microphase separation of the low molecular weight version of the BCP. Subsequent treatment of the self-assembled BCP with sequential infiltration synthesis yields sub-10 nm AlOx lines.

Automated solvent vapor annealing with nanometer scale control of film swelling for block copolymer thin films

Molecular self-assembly of block copolymers has been pursued as a next generation high-resolution, low-cost lithography technique. Solvent vapor annealing is a promising way of achieving self-assembled patterns from polymers with high interaction parameters, χ, or high molecular weights. Compared to thermal annealing, the assembly in a solvated state can be much faster, but the film swelling process is typically challenging to control and reproduce. We report the design and implementation of an automated solvent annealing system that addresses these issues. In this system the film swelling is controlled via local heating or cooling, which enables exceptionally fast and precise modulation of the swelling. The swelling of the polymer films follows preprogrammed annealing profiles with the help of a feedback loop that compares and tunes the film thickness with respect to the set point. The system therefore enables complex annealing profiles such as rapid cyclic swelling and deswelling. We show that the orientation of the pattern morphology and the amount of lattice defects are influenced by the used annealing profile. We demonstrate that optimized profiles significantly shorten the annealing time (<15 min) of high-χ and high-molecular weight poly(styrene-b-2-vinylpyridine).

Design and Testing of a Bending-Resistant Transparent Nanocoating for Optoacoustic Cochlear Implants

A nanosized coating was designed to reduce fouling on the surface of a new type of cochlear implant relying on optoacoustic stimulation. This kind of device imposes novel design principles for antifouling coatings, such as optical transparency and resistance to significant constant bending. To reach this goal we deposited on poly(dimethylsiloxane) a PEO-based layer with negligible thickness compared to the curvature radius of the cochlea. Its antifouling performance was monitored upon storage by quartz crystal microbalance, and its resistance upon bending was tested by fluorescence microscopy under geometrical constraints similar to those of implantation. The coating displayed excellent antifouling features and good stability, and proved suitable for further testing in real-environment conditions.

Temperature-Controlled Solvent Vapor Annealing of Thin Block Copolymer Films

Solvent vapor annealing is as an effective and versatile alternative to thermal annealing to equilibrate and control the assembly of polymer chains in thin films. Here, we present scientific and practical aspects of the solvent vapor annealing method, including the discussion of such factors as non-equilibrium conformational states and chain dynamics in thin films in the presence of solvent. Homopolymer and block copolymer films have been used in model studies to evaluate the robustness and the reproducibility of the solvent vapor processing, as well as to assess polymer-solvent interactions under confinement. Advantages of utilizing a well-controlled solvent vapor environment, including practically interesting regimes of weakly saturated vapor leading to poorly swollen states, are discussed. Special focus is given to dual temperature control over the set-up instrumentation and to the potential of solvo-thermal annealing. The evaluated insights into annealing dynamics derived from the studies on block copolymer films can be applied to improve the processing of thin films of crystalline and conjugated polymers as well as polymer composite in confined geometries.

All-Metal Broadband Optical Absorbers Based on Block Copolymer Nanolithography

The growing interest in solar energy during recent years has spurred on the development of high-efficiency optical absorbers using emerging concepts in plasmonics and metamaterials. Most absorber designs require patterning on a subwavelength scale, making large-scale fabrication expensive or impractical. This study presents an all-metal metasurface with tightly packed, sub-80 nm nanodomes fabricated by template-stripping thin gold films from reusable silicon templates. Subwavelength patterning was achieved via molecular self-assembly of block copolymers, which enables large-area, periodic patterning with nanometer precision. The proposed nanodome surface acts as an optical absorber capable of absorbing 97% of incident light in the visible range 320-650 nm, and still more than 90% at high incidence angles. We demonstrate both experimentally and theoretically that the absorption behavior of the thin film can be controlled by changing the size of the nanodomes, namely, the gap between the structures. The enhanced absorption of light is attributed to localized particle plasmon and gap plasmon resonances. This research provides a straightforward and cost-effective strategy to design and fabricate thin, large-area, light-absorbing coatings that can be transferred onto nearly any rigid or flexible substrate. The all-metal metasurfaces are a promising candidate for plasmon-induced hot electron generation for efficient solar energy conversion in photovoltaic and photocatalytic devices.

Nitrogen reactive ion etch processes for the selective removal of poly-(4-vinylpyridine) in block copolymer films

Self-assembling block copolymer (BCP) patterns are one of the main contenders for the fabrication of nanopattern templates in next generation lithography technology. Transforming these templates to hard mark materials is key for pattern transfer and in some cases, involves selectively removing one block from the nanopattern. For poly(styrene)-block-poly(4-vinylpyridine) (PS-b-P4VP), a high χ BCP system which could be potentially incorporated into semiconductor nanofabrication, this selective removal is predominantly done by a wet etch/activation process. Conversely, this process has numerous disadvantages including lack of control and high generation of waste leading to high cost. For these reasons, our motivation was to move away from the wet etch process and optimise a dry etch which would overcome the limitations associated with the activation process. The work presented herein shows the development of a selective plasma etch process for the removal of P4VP cores from PS-b-P4VP nanopatterned film. Results have shown that a nitrogen reactive ion etch plasma has a selectivity for P4VP of 2.2:1 and suggest that the position of the nitrogen in the aromatic ring of P4VP plays a key role in this selectivity. In situ plasma etching and x-ray photoelectron spectrometry measurements were made without breaking vacuum, confirming that the nitrogen plasma has selectivity for removal of P4VP over PS.

Straightforward Integration Flow of a Silicon-Containing Block Copolymer for Line-Space Patterning

A promising alternative for the next-generation lithography is based on the directed self-assembly of block copolymers (BCPs) used as a bottom-up tool for the definition of nanometric features. Herein, a straightforward integration flow for line-space patterning is reported for a silicon BCP system, that is, poly(1,1-dimethylsilacyclobutane)-b-poly(styrene) (PDMSB-b-PS), able to define sub 15 nm features. Both in-plane cylindrical (L₀ = 20.7 nm) and out-of-plane lamellar structures (L₀ = 23.2 nm) formed through a rapid thermal annealing-10 min at 180 °C-were successfully integrated using graphoepitaxy to provide a long-range ordering of the BCP structure without the use of underlayers or top coats. Subsequent deep transfer into the silicon substrate using the hardened oxidized PDMSB domains as a mask is demonstrated. Combining a rapid self-assembly behavior, straightforward integration, and an excellent etching contrast, PDMSB-b-PS may become a material of choice for the next-generation lithography.

Large Block Copolymer Self-Assembly for Fabrication of Subwavelength Nanostructures for Applications in Optics

Nanostructured surfaces are common in nature and exhibit properties such as antireflectivity (moth eyes), self-cleaning (lotus leaf), iridescent colors (butterfly wings), and water harvesting (desert beetles). We now understand such properties and can mimic some of these natural structures in the laboratory. However, these synthetic structures are limited since they are not easily mass produced over large areas due to the limited scalability of current technologies such as UV-lithography, the high cost of infrastructure, and the difficulty in nonplanar surfaces. Here, we report a solution process based on block copolymer (BCP) self-assembly to fabricate subwavelength structures on large areas of optical and curved surfaces with feature sizes and spacings designed to efficiently scatter visible light. Si nanopillars (SiNPs) with diameters of ∼115 ± 19 nm, periodicity of 180 ± 18 nm, and aspect ratio of 2-15 show a reduction in reflectivity by a factor of 100, <0.16% between 400 and 900 nm at an angle of incidence of 30°. Significantly, the reflectivity remains below 1.75% up to incident angles of 75°. Modeling the efficiency of a SiNP PV suggests a 24.6% increase in efficiency, representing a 3.52% (absolute) or 16.7% (relative) increase in electrical energy output from the PV system compared to AR-coated device.